120 REACTIONS INVOLVED IN [CH. 



behaviour of anthocyanin in alcohol further confirms the view that 

 the pigment is formed by oxidation from a colourless chromogen. 

 According to these authors when coloured petals of stocks (Matthiola) 

 are placed in absolute alcohol, some colour is extracted by the alcohol, 

 but both petals and alcohol become colourless in the course of an hour 

 or so. If now the colourless petals are removed from alcohol and placed 

 in water, the colour returns, and more rapidly if the water be hot. 

 Keeble, Armstrong & Jones explain these phenomena in the following 

 way. Anthocyanin is formed from a colourless chromogen by oxidation, 

 the agent being an oxidase which can only act in the presence of water ; 

 there is present also a reducing agent (which is not an enzyme), and 

 this reduces anthocyanin to its colourless chromogen when the action 

 of the oxidase is inhibited by absolute alcohol. The validity of this 

 explanation has been questioned both by Wheldale & Bassett (621) and 

 Tswett (252). The two former authors note that an extract of antho- 

 cyanin made in boiling absolute alcohol (which cannot according to 

 Keeble & Armstrong contain oxidase) loses its colour on standing. 

 The colour can be brought back by dilution with water. Hence 

 obviously no oxidase is necessary. Moreover colour can be restored 

 to the alcohol solution by dry hydriodic acid gas, a powerful reducing 

 agent. Colour is also restored to petals, decolorised in alcohol, when 

 they are placed in boiled water through which a stream of hydrogen 

 has passed for some time and still continues to pass ; a condition under 

 which any leuco-compound should be stable. Similar criticisms to 

 these are also offered by Tswett. 



We now turn to the second class of evidence. (2) Presence of 

 chromogen. 



Evidence for the presence or absence of chromogens is given by 

 Jones (237). According to this author four classes of white flowers 

 may be distinguished, (a) Those, such as Lychnis coronaria, Anemone 

 japonica, Chrysanthemum sp. which produce a brown or brownish-red 

 pigment when subjected to alcohol, chloroform, etc. Such flowers 

 contain a direct oxidase ; (b) those, such as varieties of Dianthus Caryo- 

 phyllus and D. barbatus, which give similar pigments only on addition 

 of hydrogen peroxide and contain peroxidase only; (c) those, such as 

 white varieties of Plumbago capensis and Swainsonia Tacsonia, which 

 do not produce a brown pigment even after addition of hydrogen peroxide 

 but contain a peroxidase, and (d) those, such as varieties of Sweet 

 William mentioned above under (1) which have no peroxidase and 

 hence no oxidase. Reference to the account previously given of 



